1. Field of Invention
The present invention relates to a holographic storage and reproduction system and method, and more particularly, to a holographic storage and reproduction system and method with a servo function.
2. Related Art
In the current optical storage media market, the capacity of a commercialized blue-ray disc rarely exceeds 100 GBytes, so various possible recording techniques for ultra high capacity are widely being developed, wherein the holographic optical disc has the greatest potential. The holographic recording technique has been developed for a long time, but it cannot be applied to a consumer optical storage product due to many factors. For example, early holographic experiments required a huge laser source with a high power of more than hundreds of milliwatts, a complex optical system, and a ponderous anti-vibration table, and moreover a photo-refractive crystal typically used as a holographic recording medium which is much more expensive than other ordinary media. However, along with the progress of the technique and new ideas, the limits placed on the holographic storage recording technique have been removed one by one. For example, a miniaturized high power laser, a high sensitive recording material, and a small-scale data access optical system with a position servo function have already been developed, and the conventional thought that a recording medium should be rewritable has been broken by consumer behavior in the optical disc market. Nowadays, it is still difficult for recording material of rewritable photo-refractive crystal to satisfy conditions such as a good material property, high data security, and cheap price. Illuminated by the popularization of single-write multi-read CD-R/DVD-Rs and other ordinary optical discs, the cognition that a holographic recording medium can be made of material other than rewritable material has become widely accepted. If the rewrite function is not taken into consideration, many cheap and highly sensitive organic materials can be used as the data-recording layer of a holographic optical disc, such as photopolymers. Photopolymers may generate a chemical reaction similar to molecular bonding under an irridation of strong recording light, and therefore, three-dimensional holographic interference fringes of data can be recorded and a data reproduction can be carried out through the change of optical properties caused by the molecular bonding density distribution. The concept of the aforementioned small-scale data access optical system with a position servo function is derived from a servo mechanism of the CD/DVD optical disc driver, and it is a critical factor for the holographic optical disc to be practical.
A holographic storage technique, as described in U.S. Patent Gazette Publications NO. 20040212859 and NO. 6700686, is applicable in a transmissive holographic recording medium. Due to a transmissive design, an image sensor is disposed at the opposite side of the holographic recording medium, such that the whole volume of the system becomes larger. Generally, in such transmissive system architecture design, the optical axis of the objective lens through which the signal beam passes is selected to be perpendicular to the holographic recording medium, and the reference light should be incident to the holographic recording medium in an oblique direction. Thus, a deviation is easily caused for a relative position and direction between the reference light and the holographic recording medium. Once the deviation is generated, when the reference light cannot be incident to the holographic recording medium along an original path, no reproduced signal light will be formed; thus the reproduced signal light cannot be captured by adjusting the signal light path, the image sensor for receiving reproduced optical signals will obtain no reproduced optical signal, and of course no correctly reproduced data can be reverted by an image processing technique. For a static holographic recording medium, if such an architecture enables the reference light to make a small scale scan on direction and position, the reproduced optical signal can be obtained, however, for a continuously moving holographic recording medium, the reproduced optical signal is still hard to be obtained.
Another related technique, as described in U.S. Patent Gazette Publications No. 6721076 and No. 6909529, provides an optical architecture applicable in a reflective holographic recording medium, however, no specific servo method is provided.